Definition
A precision machining process in which a long boring bar, supported at both ends, is passed through a series of in-line bearing bores in an engine case to cut them so they share the same centerline and the correct diameter. It is used during overhaul to restore main bearing bores in a crankcase or camshaft bores so they are perfectly aligned and round.
Plain English
Cutting a row of holes in an engine case in one straight pass so they all line up perfectly and are exactly the right size.
Context Anchor
Seen in engine overhaul and repair work, especially when openings in an engine case must be brought back into exact alignment.
Derivation
‘Bore’ comes from Old English ‘borian,’ meaning to pierce or make a hole. ‘Line’ refers to the straight axis the holes share. Together: boring a series of holes along one straight line.
Why Pilots Care
Proper main-bearing alignment prevents crankshaft binding, reduces vibration, and extends engine life; misalignment can lead to rapid bearing wear or catastrophic engine failure.
Intuition Check
Do not read boring as uninteresting. In maintenance, boring means cutting or finishing a round hole; line boring means doing that so several holes line up on one straight path.
Example Sentence 1
After welding the cracked crankcase, the shop sent it out for line boring to bring the main bearing bores back into alignment.
Example Sentence 2
The maintenance manual required line boring whenever the main bearing bores showed more than the allowable out-of-alignment tolerance.